The progress of a high-level information-oriented society in recent years is remarkable, and the development of digital technologies has led to the penetration of computers and communication technologies such as computer networks in everyday life. Keeping in step with this penetration, flat-screen TV sets and notebook-size personal computers have become increasingly popular, resulting in an increasing demand for displays such as liquid crystal displays, organic EL displays and electronic paper displays. Especially in recent years, there is an outstanding move toward larger displays of higher definition, leading to an ever increasing number of pixels. It is, therefore, necessary to assemble numerous field-effect transistors corresponding to the number of pixels. In a liquid crystal display, the liquid crystal is driven by providing the respective pixels with field-effect transistors as active elements and performing ON/OFF control of signals.
As field-effect transistors for use as active elements, thin-film transistors can be used. The performance of the thin-film transistors is determined by the semiconductor material and structure employed therein. In their performance, the availability of particularly high carrier mobility and high ON/OFF ratio makes it possible to obtain a large current, thereby enabling not only to drive an organic EL device or the like but also to miniaturize the thin-film transistors and to provide an improved contrast.
For thin-film transistors useful as active elements, a silicon-based semiconductor material such as amorphous silicon or polysilicon can be used as an inorganic semiconductor material. In this case, a thin-film transistor is fabricated by forming such a silicon-based semiconductor material in a multilayered structure such that source, drain and gate electrodes are successively formed on a substrate.
For the fabrication of thin-film transistors making use of the above-described silicon-based semiconductor material, however, large-scale and costly fabrication facilities are needed, and because of the use of photolithography, many process steps have to be gone through, resulting in a practical problem that the fabrication cost becomes higher. Furthermore, the fabrication requires high temperatures of from 300° C. to 500° C. or even higher, which lead not only to still higher fabrication cost but also to a problem that inorganic semiconductor layers can be hardly formed on plastic substrates or flexible plastic films.
On the other hand, organic thin-film transistors, which make use of thin organic semiconductor films comprised of an organic semiconductor material, are fabricated by a vapor deposition process (vacuum film-forming process) or a solution coating process (film printing process), and have the possibility of lower cost, larger area and lighter weight. Further, thin organic semiconductor films can be formed at a lower temperature compared with inorganic semiconductor layers, and therefore, can achieve cost reduction in this respect. In addition, such thin organic semiconductor films can be formed on plastic substrates or flexible plastic films, and therefore, can achieve weight reduction, and therefore, they can also be applied to flexible electronic devices and the like.
For the above-described merits, many organic semiconductor materials have been studied to date, and those making use of low-molecular compounds or conjugated high-molecular compounds as thin organic semiconductor films are known. Nonetheless, the conjugated high-molecular compounds are not considered to be fully satisfactory in performance when formed into organic thin-film transistors, although they have excellent solubility in solvents and enable to form thin organic semiconductor films by a simple solution coating process. The low-molecular compounds, on the other hand, exhibit high performance as organic thin-film transistors, but are accompanied by a problem in that they have poor solubility in solvents and can be hardly formed into thin films. As a method for producing a thin organic semiconductor film, the formation of the thin semiconductor film by a vapor deposition process or the formation of the thin organic semiconductor film by a solution coating process, which makes use of a dilute solution, can be mentioned. It would be very convenient if it would be possible to form a thin organic semiconductor film especially by the simple solution coating process out of these two processes. However, the solution coating process involves a problem in that, because a thin film is formed with a dilute solution of such a compound as dissolved in a solvent, it is difficult to stably obtain a film thickness sufficient to obtain stable performance as an organic transistor. An organic semiconductor material of high solubility is desired accordingly. However, high solubility and high performance are in a trade-off relationship, and a material for thin semiconductor films, said material being equipped with both high solubility and high performance, has not been developed yet.
Semiconductor materials include n-type semiconductor materials for obtaining n-type semiconductors and p-type semiconductor materials for obtaining p-type semiconductors, and for the reasons to be mentioned below, there is a long-awaited desire for the development of materials capable of exhibiting high performance especially as n-type semiconductor materials. In an n-type semiconductor material, electrons move as main carriers to produce an electric current. In a p-type semiconductor material, on the other hand, holes move as main carriers to produce an electric current. Pentacene materials and thiophene materials, which are known as organic semiconductor materials that exhibit high performance, are semiconductor materials that exhibit p-type characteristics. However, reports on n-type organic semiconductor materials of high performance are limited. For further developments of organic electronics, lower power consumption, simpler circuits and the like are essential, and therefore, organic complementary MOS circuits which require both n-type and p-type organic semiconductor materials, such as complementary metal-oxide semiconductors (CMOS), are needed. There is, accordingly, an ever-increasing desire for n-type organic semiconductor materials of high performance.
As n-type organic semiconductor materials, naphthalene imide, naphthalene diimide, and derivatives thereof are known to date. However, none of these n-type organic semiconductor materials have been reported to have high performance as thin-film transistors.
On the other hand, Non-patent Document 1 describes a low-molecular compound having the perylene skeleton and an electron mobility of 0.6 cm2/Vs, and makes mention about the possibility that its use in an organic thin-film transistor makes it possible to exhibit high performance (Non-patent Document 1).
As to organic thin-film transistors making use of perylene tetracarboxylic acid derivatives, there are those to be described below. Patent Document 1 describes that a thin film transistor comprised of an organic semiconductor material, which contains a perylene tetracarboxylic diimide derivative having a carbocyclic or heterocyclic aromatic ring system substituted with fluorine-containing groups, has a mobility of from 0.05 to 0.2 cm2/Vs and an ON/OFF ratio of from 104 to 105 and exhibits stability in air and excellent reproducibility. Patent Document 2 describes that a thin film transistor comprised of an organic semiconductor material layer, which contains a perylene tetracarboxylic diimide derivative having substituted or unsubstituted phenylalkyl groups, has a mobility of from 0.04 to 0.7 cm2/Vs and an ON/OFF ratio of from 104 to 105 and exhibits stability in air and excellent reproducibility.
Thin organic semiconductor films formed by a process such as the above-mentioned vacuum film-forming process or film printing process (solution coating process) generally have a polycrystalline structure formed of minute crystals aggregated together. Such thin organic semiconductor films contain numerous grain boundaries and defects. These crystal grain boundaries and defects act as a cause of inhibition of the transport of charges. The vacuum film-forming process and film printing process are, therefore, difficult to uniformly form a thin organic semiconductor film over a large area. These processes have hence been practically difficult to fabricate organic semiconductor devices having stable device performance.
To overcome such problems, the present inventors have already made a proposal as will be described below. Described specifically, the present inventors have proposed an organic thin-film transistor, which makes use of a thin organic semiconductor film produced by a vacuum film-forming process from N,N′-ditridecyl-3,4,9,10-pelylene dicarboxylic acid imide, an organic semiconductor material having a thermotropic liquid crystal phase at and below its decomposition temperature, and subjected to heat treatment in a temperature range in which the organic semiconductor material presents a smectic liquid crystal phase (Non-patent Document 2: 2.1 cm2/Vs electron mobility). The present inventors have then proceeded with a further study on the solubilization of perylene compounds to form thin organic semiconductor films by a solution coating process and the application of the thin organic semiconductor films to organic thin-film transistors.